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the blood in the gills, since in addition to the oxygen necessary for 
respiration, oxygen is required for inflation of the bladder. In less 
than 12 hours from the time of weighting, the fish has, presumably 
owing to the activity of the oxygen gland, completely regained its 
normal condition of quiescence, floating easily in the water without 
the aid of caudal movements (text-fig. 2 A). Im other words, the 
bladder has acquired an additional volume of gas sufficient to counteract 
the attached weight and appears very swollen on opening the fish. 
If the weight be removed at the end of the 12 hours, the fish 
Fig. 2. 
immediately floats to the surface and has to swim downwards 
vigorously in order to keep from the surface — an additional proof 
that gas has been secreted. 
Another feature always correlated with special activity of the 
oxygen gland is the closure of the oval. The oval in the Pollack 
is a large oval thin-walled area lying in the dorsal wall of the bladder 
somewhat behind the middle of its length and apparently in close 
contact with the posterior cardinal veins. The oval is usually widely 
open in any ordinary Pollack and is invisible to the naked eye, but 
on the Pollack being weighted and examined after two or three 
hours, the oval becomes quite apparent owing to the contraction of 
its muscles. This contraction of the oval muscles causes the thin- 
walled oval area to become more or less completely shut off from 
the general bladder cavity, so that the additional oxygen which is 
being pumped into the bladder is prevented from escaping at once 
into the blood stream, the conditions requiring that the blood shall 
during this period make good its loss of oxygen (to the bladder 
cavity) solely from the surrounding water and not take away from 
the bladder via the oval what it is supplying via the oxygen gland. 
As the bladder obtains its additional oxygen — as it becomes inflated 
and so counteracts the sinking effect of the weight — so the oval 
